Printing methods for transferring a printing substance onto a substrate by laser radiation are used, for example, for digitally printing printing inks or pastes. In what are known as laser transfer printing methods, the laser radiation from a laser arrangement is directed through a transparent carrier of a print medium, for example a paste contained in a capsule or an ink. The content of the capsule is transferred onto a substrate by means of the energy input.
For this purpose, the laser radiation is moved over the carrier by an optical device. At the same time, the output power of the laser beam source is time modulated such that printing substance is transferred only during an active phase and no printing substance is transferred during a passive phase.
The basic principle of a method of this type is known from WO 2002/092674 A1. In this case, a substrate faces a carrier that is transparent for the laser radiation and is coated. If this layer is irradiated with laser radiation through the carrier, some of the material of the layer is evaporated. This causes the unevaporated portion to be transferred onto the substrate.
DE 197 46 174 C1 discloses a method in which the printing substance is applied to a cylindrical, transparent printing forme which has a plurality of cells. The printing substance is transferred from the cells onto a printing material in that a change in volume or position is caused by an energy-releasing device, for example a laser beam source.
GB 2 173 452 A discloses an apparatus and a method for laser printing. In this case, a printing carrier is used, for example made of paper, the surface of which is coated with microcapsules which contain printing ink. A microcapsule of this type is made to burst by laser radiation impinging on said capsule. The printed image is formed for example by linear scanning, whereby the corresponding image points are produced by activation by means of laser radiation. For some image points, the laser is active while for others the emission is interrupted, depending on whether or not printing ink is intended to be transferred at the particular image point.
In contrast, an undesired effect also known as spiking, which occurs during the transient phase of a laser, proves to be unhelpful. After the emission of a laser has been interrupted for only a few milliseconds, the first pulse has a highly variable intensity, mostly characterized by a clear increase at the start known as a “spike.” In the printing process, this leads to considerably varying results, depending on whether or not the emission of the laser is interrupted.
DE 198 29 684 B4 discloses a laser arrangement in which the radiation from two intensity-controllable laser diodes is coupled into a fiber laser which is pumped from a separate pump source. For modulation, the laser diodes are actuated in phase opposition, i.e. alternately, meaning that the sum of the powers of the laser diodes is constant. The modulation occurs by controlling one of the laser diodes, while the other laser diode is used to keep the total power in the fiber laser constant. During the active phase of the laser arrangement, in which the radiation used is emitted, the first laser diode is active; during the passive phase, in which the output radiation is directed for example into a beam trap and the laser beam source is effectively switched off, the second laser diode is active. By always supplying the fiber amplifier with a constant power and thus keeping it in saturation, the undesired spontaneous emission of the fiber amplifier is completely suppressed and a high level of contrast is achieved.
Inter alia, an improvement of the described laser arrangement can be found in US 2011/0085149. This document describes that, at high power densities of the laser radiation together with a very narrow bandwidth, as is emitted by laser diodes, Brillouin scattering occurs inside a fiber amplifier, which scattering can lead to a destruction of the optical elements. This can be prevented, for example, in that periodic interruptions are imprinted on the control signal of the laser diodes, which interruptions lead to a significantly wider band emission.